1. Field of the Invention
The present invention relates, in general, to a method for preparing ethylbenzene from 4-vinylcyclohexene (hereinafter referred to as "4-VCH") through catalytic transfer hydrogenation. More particularly, the present invention is concerned with a method for preparing ethylbenzene in which 4-VCH is dehydrogenated in a hydrogen donor solvent with an oxidative dehydrogenation agent in the presence of palladium supported on active carbon as a heterogeneous catalyst.
2. Description of the Prior Art
C.sub.4 fraction which contains 1,3-butadiene in an economically isolable content is available in countries where ethylene is manufactured by steam cracking of naphtha. This C.sub.4 fraction contains by weight about 45 wt % of 1,3-butadiene and the remainder is butanes, butenes and butynes. 1,3-Butadiene from mixed C.sub.4 hydrocarbons is not capable of separation of simple distillation because boiling points of all components have very close temperature ranges, and some components form azeotropic mixtures.
All modern processes from isolating 1,3-butadiene are based on the physical principle of extractive distillation, and 1,3-butadiene is usually used for the production of synthetic rubbers. During the extractive distillation process, storage, or transfer, 1,3-butadiene is dimerized into 4-VCH through [2+4] Diels-Alder reaction. Currently, the produced amount of 4-VCH is approximately 1% of that of 1,3-butadiene and the whole quantity thereof is burnt. Since the by-product, 4-VCH, is increased with 1,3-butadiene, the industrial use of 4-VCH is becoming a new field of research.
Up to the present, 4-VCH had been recognized industrially as an almost useless compound. In the 1970s, there was research for the preparation of ethylbenzene or styrene from 4-VCH using oxidative dehydrogenation. However, since 1,3-butadiene, the raw material of 4-VCH, was in short supply owing to the great demand for synthetic rubbers, and moreover was expensive. The research for the oxidative dehydrogenation of 4-VCH was shown in only some documents, including U.S. Pat. Nos. 4,163,761 (1979), 4,233,244 (1980), 4,246,202 (1981), 4,300,010 (1981), 4,322,566 (1982), 4,339,622 (1982) and 4,375,571 (1983). Since the mid-1980s, there has been little research done on this topic.
More than 90% of 1,3-butadiene has been used to produce synthetic rubbers. Recently, the demand for synthetic rubbers has grown dull, but the supply of 1,3-butadiene has increased. Accordingly, [2+4] Diels-Alder reaction and dehydrogenation for 1,3-butadiene are now being actively studied with the aim of utilizing 1,3-butadiene for purposes other than synthetic rubbers, as disclosed in Applied Catalysis, 47(1989), L7-L8. Particularly, active research for the dehydrogenation of 4-VCH has been done with the hope of commercial success. As evidence, many patents have been published in 1944, including U.S. Pat. No. 5,276,257, (1994), 5,300,719 (1994), 5,321,180 (1994) and 5,336,822 (1994), Japanese Pat. Laid-Open Publication Nos. Heisei 6-329563 and Heisei 7-41436, and PCT Nos. 94/01385 and 94/29248. It is highly expected that the dehydrogenation of 4-VCH would contribute to the utilization of 1,3-butadiene, which is anticipated to be in excess supply, as well as 4-VCH. 4-VCH is prepared from 1,3-butadiene, as disclosed in U.S. Pat. Nos. 5,096,870 (1992), 5,196,621, (1993) and 5,329,057 (1994), Japanese Pat. Laid-Open Publication Nos. Heisei 6-172238 (1994), Heisei 6-247880 (1994), Heisei 7-48288 (1995), Heisei 7-48289 (1995), and Heisei 7-48290 (1995), and PCT 94/008925. With greater progress in theory, the preparation methods of 4-VCH disclosed in these recent patents have significant advantages over those of the 1970s and can be practically applied to industrial production.
It is well known that, like cyclohexene, 4-VCH can be used as a hydrogen donor in catalytic transfer hydrogenation. Particularly, while serving as a hydrogen donor, 4-VCH is converted into ethylbenzene, 4-ethylcyclohexene, ethylchlorobenxane, styrene, and so on, through hydrogenation-dehydrogenation. An illustrative application of 4-VCH as a hydrogen donor is disclosed in U.S. Pat. No. 4,322,556 (1982), in which a catalytic transfer hydrogenation proceeds in the presence of iridium compound IrCl(CO)(Ph.sub.3 P).sub.2, a homogeneous catalyst, to reduce nitrobenzene into aniline.
The reaction of this patent is a homogeneous reaction which needs to react in high temperature. Further more, because the homogeneous catalyst used is composed of a precious metal, the catalyst needs to be recovered. In addition, the reaction causes a serious problem of heavy metal contamination. In fact, because environmental pollution is more serious problems than production at the present time, its industrial production is virtually impossible.
The preparation of aromatic compounds through the dehydrogenation of 4-VCH can be accomplished by either a liquid phase reaction using nitrobenzene or a gas phase reaction using metal of metal oxide. The gas phase reaction is reported in U.S. Pat. No. 5,276,257 (1994) in which ethylbenzene or styrene is prepared from 1,3-butadiene in the presence of a heterogeneous catalyst containing molybdenum. For heterogeneous catalyst, the oxide of magnesium, zinc, calcium, strontium and/or barium is also reported in U.S. Pat. No. 5,300,719 (1994). Besides these, U.S. Pat. No. 5,336,822 (1994) also discloses the preparation of styrene from 4-VCH in the presence of a heterogeneous catalyst comprising antimony by using oxygen.
Such a gas phase reaction is a heterogeneous reaction in which metals or metal oxides are used to obtain high conversion rates while maintaining high reaction temperature. Through the gas phase reaction, ethylbenzene or styrene can be obtained by subjecting 4-VCH to oxidative dehydrogenation. Such oxidative dehydrogenation through gas phase reaction is superior in reactivity and selectivity to liquid phase reaction. However, reaction temperature should be increased according to the short contact period between the catalyst and reactants. High reaction temperatures results in the short lifetime of catalysts and a lot of by-products.